<!DOCTYPE html>
<html>
<head>
  <style>
    .custom-loader {
      position: fixed;
      top: 0;
      left: 0;
      width: 100vw;
      height: 100vh;
      background: red;
      display: flex;
      justify-content: center;
      align-items: center;
      z-index: 9999;
    }
    .spinner {
      width: 50px;
      height: 50px;
      border: 5px solid #f3f3f3;
      border-top: 5px solid #3498db;
      border-radius: 50%;
      animation: spin 1s linear infinite;
    }
    @keyframes spin {
      0% { transform: rotate(0deg); }
      100% { transform: rotate(360deg); }
    }
  </style>
</head>
<body>
  <div class="custom-loader">
    <div class="spinner"></div>
  </div>
</body>
</html>